Overwhelmingly, commodity, intermediate and specialty organic chemicals used throughout the world are derived, ultimately, from crude oil by processes involving chemical catalysis. Crude oil is first refined, typically by catalyzed steam cracking, into hydrocarbon intermediates such as ethylene, propylene, butadiene, benzene, and cyclohexane. These hydrocarbon intermediates then typically undergo one or more catalytic reactions by various processes to produce the desired chemical(s).
Among the plethora of intermediate and specialty chemicals derived from crude oil is adipic acid. Adipic acid is an important industrial chemical, the primary use of which is as a monomer in the production of nylon 6,6. Other significant uses of adipic acid include use in the production of urethanes, diesters and polyesters. Simplistically viewed, current commercial scale production of adipic acid involves refining crude oil to produce cyclohexane, followed by the selective catalytic oxidization of cyclohexane into “KA oil” which, in turn, is further oxidized in the presence of nitric acid to produce adipic acid.
1,6-Hexanediol is a valuable, specialty chemical. It is currently used in the synthesis of a variety of polymers and other specialty products such as, for example, urethane foams, elastomers, coatings, adhesives and plasticizers. 1,6-Hexanediol is produced industrially by the catalytic hydrogenation of adipic acid or its esters. Mixtures of adipic acid, hydroxycarboxylic acids with other C6 components formed in, for example, the above-mentioned cyclohexane oxidation process can also be used. Typically, adipic acid or such mixtures are hydrogenated continuously in the presence of catalysts comprising cobalt, copper or manganese. Hydrogenation processing conditions include reaction temperatures in the range of about 170-240° C. and pressures in the range of about 15.0-30.0 MPa. These hydrogenation reactions have been conducted in trickle-flow (down-flow) or bubble-flow (up-flow) fixed-bed reactors. The crude reaction product from this hydrogenation reaction typically includes not only 1,6-hexanediol but also other alcohols, ethers, other diols, and esters. The 1,6-hexanediol is typically recovered by fractional distillation of the crude reaction product. If esters of adipic acid are employed as the substrate to produce 1,6-hexanediol, supported catalysts such as copper chromite or copper with added zinc and barium have been used. Ruthenium, platinum, or palladium on inert supports have also been used. Gas-phase hydrogenation of esters of adipic acid has been carried out at pressures in the range of about 1-7 MPa.
The production of 1,6-hexanediol from adipic acid hinders the prospects of utilizing hexanediol as a building block chemical for the production of large volume valuable chemicals because of the stand-alone commercial value of the feedstock, adipic acid. In support of that proposition, it is notable that the current market for adipic acid is almost 6 billion lbs/annum, but the current worldwide production of 1,6-hexanediol is only on the order of about 250M lbs/annum.
For many years there has been an interest in using renewable materials as a feedstock to replace or supplement crude oil derived basic chemicals as the feedstock for the production of intermediate and specialty chemicals. See, for example, Klass, Biomass for Renewable Energy, Fuels, and Chemicals, Academic Press, 1998, which is incorporated herein by reference. Given the steep rise in the price of oil and its price volatility, there is an ever increasing interest in shifting away from the utilization of conventional, oil-derived starting materials. Recently, processes for the production of 1,6-hexanediol from fructose via 5-hydroxymethylfurfural have been disclosed in WO2011/149339, although the disclosed processes appear to suffer from very low yields. Also disclosed therein are processes for converting 1,6-hexanediol to large volume chemicals such as caprolactam.
If processes for the production of 1,6-hexanediol from renewable feedstock at a cost less than the current cost for producing the same from oil-derived adipic acid could be commercialized, 1,6-hexanediol could become a vital building block chemical the uses for and production volume of which would expand exponentially.
U.S. Pat. No. 4,400,468 discloses a process for producing adipic acid from a renewable resource, specifically, biomass such as waste material selected from paper, wood, cornstalks and logging residue. The process involves oxidizing 1,6-hexanediol in the presence of a microorganism, such as Gluconobacter oxydans subsp. oxydans, to produce adipic acid. No examples are disclosed in this patent regarding any yields obtainable from this process.
In light of the changing environment toward utilization of cheaper, renewable feedstocks, the discovery of new, industrially scalable methods for the selective and economical production of adipic acid from 1,6-hexanediol could have extraordinary value in the near future.